Protected remote alarm system



Nov. 3, 1964 J. G. w. LEE ETAL 3,l55,953

PROTECTED REMOTE ALARM SYSTEM Filed sept. 27, 1980 DELA Y* `26 omsRAm/e I I I I I I I I I I I I I I L United States Patent O 3,155,953 PROTECTED REMOTE ALARM SYSTEM John G. W. Lee, Weiwyn, and Lionel R. F. Thompson, Hatfield, England, assignors, by mesne assia'gnments5 to The De Havilland Aircraft Company Limited, Hattield, England, a company of Great Britain Filed Sept. 27, 19:50, Ser. No. 58,836 Claims priority, application Great Britain, Get. 15, 1959, 34,922/59 6 Claims. (Cl. Mii- 253) This invention relates to remote alarm systems.

Such a remote alarm system is intended to initiate a remote alarm Circuit upon failure of an electrical circuit. The electrical circuit may be part of a security system, which is broken by operation of a detector system, for example an accelerometer system Sensitive to attacks upon strong rooms or a system sensitive to the interruption of light radiations.

The security electrical circuit includes a coil of a normally energised relay which holds a switch closed. The switch forms part of a transmission Circuit between the security system and the alarm circuit. The latter includes a switch which is held open by a relay whose energisation is broken upon opening of the transmission circuit. The switch held closed by the relay is in the vicinity of the security system at one end of the transmission circuit and the switch held open by the relay is in the vicinity of the alarm circuit at the other end of the transmission circuit.

The transmission circuit may include two external wires running from the location of the security system to the location of the alarm circuit. Such wires being accessible may be tampered With in order to prevent operation of the alarm circuit even when the security circuit is broken.

Thus, for example, the switch held closed by the relay may be shunted by shorting the external wires whereafter failure of energisation of the security system relay will not efiect initiation of the alarm circuit.

According to the invention, electrical energisation of the relay which holds open the alarm circuit switch is derived from two electrical pulse generators, one of which is Controlled by the other through a preset delay circuit which includes a resistor and a capacitor in a time constant OR. circuit, the capacitor and resistor being located at opposite ends of the transmission circuit.

Preferably, one of the capacitor and the resistor is located in the vicinity of the electrical circuit and the other in the vicinity of the generators.

Advantageously, the capacitor is located in the vicinity of the electrical circuit and the resistor in the vicinity of the generators.

One electrical pulse generator may have a mark space ratio of approximately one-to-one in which case the delay is equivalent to one mark or an odd number of marks.

The scope of the invention is defined by the appended claims; and how it can be carried into effect is hereinafter particularly described with reference to the accompanying drawing which shows diagrammatically an alarm system according to the present invention.

The security system 10 includes an electrical circuit which is intended to be broken by the occurrence of an event which is to be indicated by initiation of an alarm circuit 12. The security electrical circuit includes a relay whose coil 14 is normally energised when the security electrical circuit is unbroken. The coil 14 of the relay controls the operation of a switch 16 which is held Closed by energisation of the relay 14. The switch 16 is connected in series with a capacitor 18 and across two external wires 28 and 22 which extend from the vicinity of the security system. The wires 20 and 22 may extend for considerable distance in the open to the vicinity of the 3,l55,953 Patented Nov. 3, 1964 alarm circuit 12. One Wire 20 leads to one end of a resistor 24 in series with a capacitor 18, the other end of the resistor being connected to a delay generator circuit 26 of which the resistor 24 and capacitor 18 form a time constant OR. circuit. The delay generator circuit 26 receives electrical pulses from a master pulse generator 28 which is arranged to generate flat top pulses at approximately one-to-one mark space ratio. A second pulse generator 3h is Controlled from the delay generator circuit 26 so as to generate pulses similar to the master generator pulses but delayed by one mark. The outputs f both pulse generators 28 and 30 are applied to the coil 32 of a relay whose switch 34 is normally held open by energisation of the coil and forms the initiating switch of the alarm circuit 12.

In operation the pulse generators 28 and 30 which are supplied with power from two alternative sources (not shown) maintain energisation of the coil 32 because the flat top pulses provide a substantially steady D.C. potential. The delay generated in the delay generator circuit 26 is set by the capacitance and resistance of the capacitor 18 and 'resistor 24 which are chosen at the time of installation. The particular values depend upon the capacitance and resistance of the transmission circuit wires 2d and 22 but in addition may be varied from installation to installation, giving the same time Constant to ensure that the pulses derivcd from the generator 30 fill in the spaces between the pulses derived from the pulse generator 28.

If an attempt is made to tamper with the wires 20 and 22 which extend from the security system to the delay generator 26 then the capacitance of the transmission circuit external to the location of the delay generator 26 and pulse generators 23 and 3G, which location is in the vicinity of the alarm circuit, will be altered, as the value of the capacitor 18 cannot be determined without knowledge of the resistance of the resistor 24 even though the value of the product CR. may be known. Any alteration in the capacitance or resistance of the transmission circuit will alter the delay generated by the delay generator circuit 2d so that the pulses from the pulse generator 30 will not fill in the spaces between the pulses generated by the pulse generator 28. When this occurs the coil 32 ceases to be energised at all times and the relay switch 34 closes so as to initiate the alarm circuit 12.

We claim:

1. A remote alarm system comprising a monitored electric circuit, an electric switch disposed in proximity to said monitored circuit and responsive to current fiow in said monitored circuit to assume a normally conducting condition, first and second electric pulse generators disposed at a location remote from said monitored circuit, delay means including a resistor and a capacitor electrically connecting through said switch the pulse output of said first generator to said second generator to control the pulse output of said second generator at a predetermined phase displacement relative to the pulse output of said first generator, at least one of said resistor and said capacitor being located in the vicinity of said monitored circuit and alarm initiating means electrically connected to the outputs of said first and second generators and responsive both to said switch assuming the non-conducting condition and to change in said predetermined phase displacement to produce an alarm initiating signal.

2. A remote alarm system comprising a monitored electric circuit disposed at one location an electric switch disposed in proximity to said monitored circuit and responsive to current ilow in said monitored circuit to assume a normally conducting condition, first and second electric pulse generators disposed at another location remote from said monitored circuit, a delay generator and a capacitor and a resistor electrically connected in series with said switch between the output of said first generator and said second generator to control the phase displacement of the pulses generated by said second generator relative to the pulses generated by said first generator at a selected value determined at least in part by the Capacitance of said capacitor and the resistance of said resistor, said capacitor being disposed at said one location and said resistor heing disposed at said other location, and alarm initiating means electrically connected to the outputs of said first 'and second pulse generators and responsive both -to said switch assuming the non-conducting condition and to change in said selected phase displacement to produce an alarm initiating signal.

3. A remote alarm system Comprising a monitored electric Circuit at one location, a first coil connected in said monitored circuit, an alarm Circuit at another location remote from said one location, first and second pulse generators at said other location, a delay generator coupled to said second generator and operable to control the phase displacement of the output pulses of said second generator at a predetermined value with respect to `an input pulse train applied to said delay generator, a

transmission Circuit connected between said first generator and said delay generator to apply the pulse output of said first generator to said delay generator as said input pulse train, said transmission circuit including a first electrical switch disposed at said one location and responsive to current fiow in said first coil to assume a normally conducting condition, and a resistor and a capacitor electrically connected in series with said first switch, at least one of said resistor and said capacitor being disposed at said one location and the values of said resistor and capacitor determining at least in part said predetermined value of phase displacement, a second coil disposed at said other location and electrically connected to the outputs of said first and second generatora to sum the outputs thereof and a second switch included in said alarm Circuit and responsive to the summed outputs of said first and second generators in said second coil to assume the non-conducting condition When said first and second generator outputs have said predetermined phase displacement and to assume the conductng condition to complete said alarm Circuit and ini'tiate an alarm when said phase displacement departs from said predetermined value.

4. A remote alarm system according to claim 3 wherein said capacitor is disposed at said one location and said resistor is disposed at said other location.

5 A remote alarm system comprising a monitored electric circuit at one location, a first coil connected in said monitored Circuit, and alarm circuit at another location remote from said one location, first and second pulse generators at said other location and operable to produce similar' output pulse trains having a mark to space ratio of unity, a delay generator coupled to said second generator and opcrable to control the phase displacement o-f the output pulse train of said second generator at a predetermined value with respect to an input pulse train applied to said delay generator, a transmission Circuit connected between said first generator and said delay generator to apply the pulse train generated by said first generator to said delay generator as said input pulse train, said transmission Circuit including a first electrical switch disposed at said one location and responsive to current flow i'n said Vfirst coil to assurne a normally conducting condition, and

a resistor and a capacitor electrically connected in series With said first switch, lsaid Capacitor being at said one location and said resistor being at said other location and said resistor and capacitor having values selected to control said predetermined phase displacement at a value corresponding to an odd number of marks, a second coil disposed at said other location and supplied With the output pulse trains of said first and second generators to sum said output pulse trains and produce a substantially DC. signal for a phase displacement of said odd number of marlts and a second switch included in said alarm cir- Cuit and normally maintained in the non-conducting condition by the DC. signal in said second coil and being responsive to interruption of said D.C. signal to complete said alarm circuit and initiate an alarm' 6. A remote alarm system according to claim 5 where- 'in the values of said resistor and said capacitor are selected to produce a phase displacement of one mark.

References Cited in the file of this patent UNITED STATES PATENTS 2,088,295 Hubble July 27, 1937 V2,454,807 Kennedy Nov. 30, 1948 2,928,081 GrossoV Mar. 8, 1960 

1. A REMOTE ALARM SYSTEM COMPRISING A MONITORED ELECTRIC CIRCUIT, AN ELECTRIC SWITCH DISPOSED IN PROXIMITY TO SAID MONITORED CIRCUIT AND RESPONSIVE TO CURRENT FLOW IN SAID MONITORED CIRCUIT TO ASSUME A NORMALLY CONDUCTING CONDITION, FIRST AND SECOND ELECTRIC PULSE GENERATORS DISPOSED AT A LOCATION REMOTE FROM SAID MONITORED CIRCUIT, DELAY MEANS INCLUDING A RESISTOR AND A CAPACITOR ELECTRICALLY CONNECTING THROUGH SAID SWITCH THE PULSE OUTPUT OF SAID FIRST GENERATOR, TO CONTROL THE PULSE OUTPUT OF SAID SECOND GENERATOR AT A PREDETERMINED PHASE DISPLACEMENT RELATIVE TO THE PULSE OUTPUT OF SAID FIRST GENERATOR, AT LEAST ONE OF SAID RESISTOR AND SAID CAPACITOR BEING LOCATED IN THE VICINITY OF SAID MONITORED CIRCUIT AND ALARM INITIATING MEANS ELECTRICALLY CONNECTED TO THE OUTPUTS OF SAID FIRST AND SECOND GENERATORS AND RESPONSIVE BOTH TO SAID SWITCH ASSUMING THE NON-CONDUCTING CONDITION AND TO CHANGE IN SAID PREDETERMINED PHASE DISPLACEMENT TO PRODUCE AN ALARM INITIATING SIGNAL. 